Metallizing compositions and the preparation of electrically conductive articles

ABSTRACT

Metallizing compositions suitable for the production of fired on electrically conductive films which do not adversely affect the overall strength of heat resistant substrates are described. The compositions comprise, in specified proportions, at least one finely divided electrically conductive metal, certain alkali metal, crystallizable, high silica glass frits as binder, inert liquid vehicle and, if desired, inert filler and/or pigment. The compositions are particularly suited to glass substrates and provide therewith composite structures having greater strength than that of the glass substrate.

This is a division of application Ser. No. 638,785, filed Dec. 8, 1975now U.S. Pat. No. 4,029,605.

This invention relates to metallizing compositions containing anelectrically conductive metal and a crystallizable frit, to a method forapplying these compositions to heat resistant substrates andparticularly glass and other ceramic substrates, and to articles of heatresistant substrates having an electrically conductive metal film fusedto at least a portion of their surfaces.

Metallizing compositions and particularly paste-like or liquidmetallizing compositions containing finely-divided silver particles andan inorganic binder dispersed in a liquid vehicle are known to the art.In such compositions the inorganic binder component serves to bond theconductive silver firmly to the substrate to which the composition isapplied and the liquid vehicle is present to form a metallizing ink,paint or paste for ease of application to the substrate in a desiredpattern.

The conventional inorganic binders for metallizing compositions whichprovide fired-on conductive metal coatings on ceramic substrates arefrits of soft glasses such as, for example, the metal borate glasses,the borosilicate glasses containing at least as much boron as silica,the borosilicate high density glasses containing high amounts of bismuthand/or lead oxides, the high barium and/or strontium oxide-containingglasses, and the like. Frits of the above types in which silica is theminor glass-forming ingredient give metallizing compositions of goodadhesion and solder acceptability. However, because the metallizingcompositions have high coefficients of thermal expansion ranging usuallyfrom about +155 to +180 × 10⁻⁷ /° C., they are not suitable for use withmany substrates and particularly with the low expansion glass-ceramicsor thin flint glass substrate due to their weakening effect on thesubstrate. The designing of satisfactory metallizing coatingcompositions which will provide good substrate wetting as well ascoefficients of expansion equal to or below that of the substrate sothat weakening does not occur has not been achieved. Thus, the searchcontinues for metallizing compositions having a broader range of utilityand better characteristics than those presently known.

Now, in accordance with the present invention, it has been found thatmetallizing compositions formulated with certain alkali metal,crystallizable high silica frits not only produce on firing a conductivemetal coating having good adherence to a wide variety of substrates butthat the resulting composite structure has a greater strength value thanthat of the non-coated substrate to which the coating is applied. Thefinding that any improvement of strength can be realized with themetallizing compositions of this invention is indeed surprising becauseit has generally been recognized in the ceramic art that weakening, notstrengthening, results when a substrate is fusion coated with a materialhaving a significantly higher coefficient of expansion.

Accordingly, the present invention relates to a conductive metalcomposition adapted to be applied to and fired on a heat resistantsubstrate to form thereon an electrically conductive, adherent filmwhich does not adversely affect the overall strength of the substrate,said composition comprising, by weight, (a) from 50 to 88% of at leastone finely-divided, electrically conductive metal; (b) from 2 to 40% ofa crystallizable glass frit binder comprising by weight 40 to 70% ofSiO₂, 10 to 31% of Al₂ O₃, 3 to 20% of Li₂ O, 2 to 15% of B₂ O₃, 0 to 4%of As₂ O₃, 0 to 5% of Na₂ O, 0 to 5% of K₂ O, 0 to 6% of Bi₂ O₃, and atleast one oxide component selected from the group consisting of 4 to 19%of ZrO₂ and 1 to 10% of TiO₂ ; (c) from 5 to 48% of an inert liquidvehicle; and (d) from 0 to 20% of an inert filler or pigment. Theinvention also relates to a method for forming an electricallyconductive film on a heat resistant substrate using the composition ofthis invention and to the articles so formed.

The crystallizable glass frits which serve as the binder component inthe conductive metal compositions of this invention are, as indicated,high silica glasses comprising as base constituents on the oxide basis

    ______________________________________                                                  Range (%) Preferred Range (%)                                       ______________________________________                                        SiO.sub.2   40-70       45-65                                                 Al.sub.2 O.sub.3                                                                          10-31       10-20                                                 Li.sub.2 O   3-20        5-10                                                 B.sub.2 O.sub.3                                                                            2-15        5-15                                                 ZrO.sub.2    4-19       10-15                                                 TiO.sub.2    1-10       2-6                                                   As.sub.2 O.sub.3                                                                          0-4         0-4                                                   Na.sub.2 O  0-5         0-5                                                   K.sub.2 O   0-5         0-5                                                   Bi.sub.2 O.sub.3                                                                          0-6         2-5                                                   ______________________________________                                    

at least one of ZrO₂ or TiO₂ being present in the above amount as anucleating agent. The frits will usually also contain at least one ofAs₂ O₃, Na₂ O, K₂ O or Bi₂ O₃ as refining agents or modifiers, in whichcase the total amount of such agents or modifiers will preferably rangefrom about 1 to about 10%.

The following frit compositions are given as examples of preferredcompositions which will crystallize β-spodumene or β-eucryptite whenfired under appropriate conditions. All thermal expansion valuesreported in the Table and the examples which follow were measuredaccording to ASTM E-228-71.

                                      Table I                                     __________________________________________________________________________              A      B      C      D      E      F                                Frit         %      %      %      %      %      %                             Constituents                                                                            Wt.                                                                              Oxide                                                                             Wt.                                                                              Oxide                                                                             Wt.                                                                              Oxide                                                                             Wt.                                                                              Oxide                                                                             Wt.                                                                              Oxide                                                                             Wt.                                                                              Oxide                         __________________________________________________________________________    SiO.sub.2 600                                                                              47.5                                                                              600                                                                              49.1                                                                              600                                                                              49.6                                                                              606                                                                              48.4                                                                              450                                                                              60.4                                                                              425                                                                              59.2                          Al.sub.2 O.sub.3                                                                        230                                                                              18.2                                                                              190                                                                              15.6                                                                              190                                                                              15.7                                                                              190                                                                              15.2                                                                              110                                                                              14.8                                                                               80                                                                              11.2                          Li.sub.2 CO.sub.3.sup.(1)                                                               520                                                                              16.8                                                                              520                                                                              17.3                                                                              490                                                                              16.6                                                                              490                                                                              16.0                                                                              112                                                                               6.1                                                                              140                                                                               7.8                          TiO.sub.2 -- --  -- --  -- --  -- --   30                                                                               4.0                                                                               30                                                                               4.2                          ZrO.sub.2 160                                                                              12.7                                                                              160                                                                              13.1                                                                              160                                                                              13.2                                                                              160                                                                              12.8                                                                              -- --  -- --                            As.sub.2 O.sub.3                                                                         15                                                                               1.2                                                                               15                                                                               1.2                                                                               15                                                                               1.2                                                                               15                                                                               1.2                                                                              -- --  -- --                            H.sub.3 BO.sub.3.sup. (2)                                                                80                                                                               3.6                                                                               80                                                                               3.7                                                                               80                                                                               3.7                                                                               80                                                                               3.6                                                                              150                                                                              11.4                                                                              170                                                                              13.4                          Na.sub.2 CO.sub.3.sup.(3)                                                               -- --  -- --  -- --   60                                                                               2.8                                                                              -- --  -- --                            Bi.sub.2 O.sub.3                                                                        -- --  -- --  -- --  -- --   25                                                                               3.3                                                                               30                                                                               4.2                          Coefficient of                                                                Thermal Expan-                                                                sion (° C. × 10.sup.-7)                                                    -50    -11    -30    -3     +13    +20                              __________________________________________________________________________     .sup.(1) Oxide component Li.sub.2 O                                           .sup.(2) Oxide component B.sub.2 O.sub.3                                      .sup.(3) Oxide component Na.sub.2 O                                      

Typically, the crystallizable frits are made by mixing the batchcomponents together, melting or smelting the mixture, pouring the meltinto water to produce a frit and then grinding the resulting frit,usually with a ball mill to provide a frit composition which will pass a325-mesh screen.

The frit may include minor amounts of impurities or optionalconstituents other than those noted above, provided, of course, that theinclusion of such constituents or the amount utilized does not detractfrom the advantages of the invention.

The composition of the invention, as indicated, contains a major amountwhich is from 50 to 88% by weight of the composition of at least oneelectrically conductive metal such as the noble metals, gold, silver,platinum, palladium, rhodium and iridium in particulate form. Silver isthe preferred noble metal and most preferably will be present in anamount from 60 to 86% by weight of the composition. The particulatemetal is generally present in its elemental form and the particlesthereof are usually in powder or flake form. Preferably, the metal is apowder which will pass through a 200 mesh or smaller screen. Particulatecompounds of the metal can also be used provided the compound can bereduced to the elemental metal during processing such as by carrying outthe firing in the presence of a reducing atmosphere.

The liquid vehicle portion of the composition functions to hold theconductive metal particles and the frit binder together and to permitthe facile application of the composition to the substrate. The liquidshould be inert to the other components of the composition and to thesubstrate to which it is applied and should not interfere with theformation of a metallic film during firing. Many liquids satisfy thesegeneral requirements and are known to perform well in metallizingcompositions. Particularly suitable are pine oil, turpentine, mineraloils, glycols, clean burning heavy bodied oils and the like. The methodof application and the thickness of the desired coating will, of course,influence the proportion and type of liquid vehicle in the composition.

The metallizing composition can also contain, if desired, a small amountand preferably up to 20% by weight of the composition of inert fillersand/or pigments. The inclusion of fillers such as silica or aluminausually results in films having improved abrasion durability. Althoughthe inclusion of pigments is generally for the purpose of enhancing thecoloration of the substrate, certain pigments can also function asfillers and in this dual capacity can be used to impart color to thecomposite structure and to modify the properties of the coating in adesired manner.

The compositions of the invention can be applied to any substrate whichretains its integrity during the firing operation such as ferrous andnon-ferrous metal substrates and ceramic substrates but is particularlysuited to the ceramic substrates such as glass, china, porcelain andglass-ceramics. The method of application of the composition to thesubstrate is conventional and typically includes such well knowntechniques as spraying, brushing, dipping, banding or screen or stencilprinting.

Following application of the composition to the substrate in the desiredpattern or design, the coating is preferably, but not necessarily, driedprior to firing. Air drying will usually suffice when the vehicle isvolatile at room temperature. Under other circumstances, dry aircurrents or mild baking at elevated temperature may be required toremove the diluent. The temperature of actual firing will vary,depending upon the maturation temperature of the frit but should bebelow the point at which the substrate will physically deform or changechemically. Usually, firing at a temperature of at least about 590° C.and which is within the tempering cycle of the substrate will fuse thefrit and effect a firm bond between the composition and the substrate.Preferably, firing is carried out for 2 to 15 minutes at from about 600to about 900° C.

The compositions of the invention are particularly useful in theproduction of electrical components since the fired on metallic films,in an electrical pattern or design on ceramic objects, are easilyconnected in electrical circuits. Among the many uses are conductors,resistors and other components in printed circuits and other electronicapplications, capacitors, glass electrodes, electroconducive heatingelements on aircraft, architectural and automobile windows to removemoisture or ice deposits, ornamental metallic coatings or designs, andthe like.

The invention is further described by reference to the followingexamples wherein all parts and percentages are by weight unlessotherwise indicated.

EXAMPLES 1 to 5

Various silver metallizing pastes were prepared by blending 236 parts ofparticulate silver as powder or flake, from 23.6 to 28 parts of acrystallizable frit or frit mixture, and 41 parts of a pine oil vehicleand subjecting the blends to the shearing action of a three-roll mill.The resulting silver pastes were screen printed in an electrical gridpattern on 1/4 inch float glass plates and in an overall wraparounddesign on 1/4 inch flint glass rods, and the printed plates and rodswere dried at 425° C. for 10 minutes and then fired at 621° C. for 7minutes. A control paste was also prepared in the manner of Example 2except that the 22.4 parts of Frit F were replaced with 22.4 parts ofthe control frit and 44.7 parts of the vehicle were used. Details of thepaste compositions of these examples and the control and the propertiesof the printed glass articles are given below in Table II.

                                      Table II                                    __________________________________________________________________________                 Ex. 1                                                                             Ex. 2                                                                             Ex. 3                                                                             Ex. 4                                                                             Ex. 5                                                                             Control                                      __________________________________________________________________________    Paste Composition                                                             (parts by weight)                                                             Silver flake --  --  118 236 --  --                                           Silver powder                                                                              236 236 118 --  236 236                                          Frit E of Table I                                                                          --  --  --  --  28  --                                           Frit F of Table I                                                                          28  22.4                                                                              28  28  --  --                                           Control frit.sup.(1)                                                                       --  1.2 --  --  --  23.6                                         Pine oil vehicle                                                                           41  41  41  41  41  44.7                                         Coefficient of thermal                                                        expansion (° C. × 10.sup.-7)                                                  +135                                                                              +149                                                                              --  --  +129                                                                              +172                                         Modulus of Rupture                                                            (p.s.i.).sup.(2)                                                                           18,701                                                                            16,949                                                                            14,130                                                                            14,013                                                                            20,515                                                                            9,020                                        Electrical resistance                                                         (Ω/in).sup.(3)                                                                       0.31                                                                              0.26                                                                              0.31                                                                              0.29                                                                              0.21                                                                              0.14                                         __________________________________________________________________________     .sup.(1) Frit of a conventional high density lead borosilicate glass          containing 66.6% PbO, 22.4% of SiO.sub.2, 8.5% B.sub.2 O.sub.3 and 2.5%       Al.sub.2 O.sub.3 and having a coefficient of thermal expansion of 75          × 10.sup.-7 /° C.                                                .sup.(2) Determined on the glass rods; modulus of rupture of non-printed      glass rods was 15,129 p.s.i.                                                  .sup.(3) Determined on the glass plates.                                 

EXAMPLES 6-9

The procedure of Example 5 was repeated except that in these examples anequal amount of frit A (Example 6), frit B (Example 7), frit C (Example8) and frit D (Example 9) was substituted for frit E. The modulus ofruptures values for the fired flint glass rods printed with the pastesof these examples ranged from 17,000 to 20,000 p.s.i. and the electricalresistance values ranged from 0.26 to 0.36 Ω/inch.

If desired, the strength of ceramic substrates and particularly glassmetallized with the compositions of this invention can be furtherincreased by staining that area of the substrate to which the conductivemetal composition is to be applied. Stains are well known andcommercially available and typically comprise a silver compound such assilver chloride, a modifier such as one or more compounds of iron orcopper, inert filler ingredients, and an inert liquid vehicle. The stainis applied to the substrate conventionally as by screen printing andthen the stained substrate, with or without drying, is fired to thematuration temperature which usually ranges from about 510° to about540° C., depending upon the particular stain employed. after removal ofany residue from the firing step, the stained substrate is nextoverprinted in the area of stain with the conductive paste and thenfired as above. This results in a metallized substrate havingoutstanding strength properties. It is also possible to incorporate thestain ingredients directly with the paste constituents and arrive atmetallized substrates of improved strength. The use of a stain incombination with the paste of Example 5 is demonstrated below.

EXAMPLES 10-12

Flint glass rods having a diameter of 0.25 inch and a length of 3 incheswere printed with a stain composed of, by weight, 9.25 of silverchloride, 15.2% of ferrous sulfide, 15.2% of cuprous sulfide, 38.4% ofsilica and 22% of pine oil vehicle by screen printing and the printedrods were dried at 425° C. for 10 minutes and then fired at 510° C. for10 minutes. The resulting stained rods were next overprinted in the areaof the stain with the paste of Example 5 and the overprinted rods weredried at 425° C. for 10 minutes and then fired at 620° C. for 7 minutes.For the sake of comparison, a flint glass rod was also treated in themanner of these examples except that no stain was used and the paste wasprinted directly onto the glass rod after firing the rod at 510° C. for10 minutes. The strength values for the metallized rods and a glass rodcontrol fired at the same temperature-time cycles as above except nostain or paste was used are reported below.

    ______________________________________                                                                  Modulus of                                          Example No.    Stain      Rupture (p.s.i.)                                    ______________________________________                                        10             light      20,129                                              11             dark       22,743                                              12             none       19,872                                              glass rod control                                                                            none       15,215                                              ______________________________________                                    

What I claim and desire to protect by Letters Patent is:
 1. An articlecomprising a heat resistant substrate having fused to at least a portionof its surface an electrically conductive metal film which does notadversely affect the overall strength of the substrate and is derivedfrom a composition comprising by weight(a) from 50 to 88% of at leastone finely-divided electrically conductive metal; (b) from 2 to 40% of acrystallizable glass frit binder consisting essentially of by weight 40to 70% of SiO₂, 10 to 31% of Al₂ O₃, 3 to 20% of Li₂ O, 2 to 15% of B₂O₃, 0 to 4% of As₂ O₃, 0 to 5% of Na₂ O, 0 to 5% of K₂ O, 0 to 6% of Bi₂O₃, and at least one oxide component selected from the group consistingof 4 to 19% of ZrO₂ and 1 to 10% of TiO₂ ; (c) from 5 to 48% of an inertliquid vehicle; and (d) from 0 to 20% of an inert filler or pigment. 2.The article of claim 1 wherein the electrically conductive metal issilver and the substrate is glass.
 3. The article of claim 2 whereinsaid portion of the surface of the glass substrate to which the metalfilm is fused is silver stained.
 4. A method for forming on a heatresistant substrate an electrically conductive metal film which does notadversely affect the overall strength of the substrate, which methodcomprises applying to at least a portion of the surface of saidsubstrate a layer of a conductive metal composition comprising byweight(a) from 50 to 88% of at least one finely-divided electricallyconductive metal; (b) from 2 to 40% of a crystallizable glass fritbinder consisting essentially of by weight 40 to 70% of SiO₂, 10 to 31%of Al₂ O₃, 3 to 20% of Li₂ O, 2 to 15% of B₂ O₃, 0 to 4% of As₂ O₃, 0 to5% of Na₂ O, 0 to 5% of K₂ O, 0 to 6% of Bi₂ O₃, and at least one oxidecomponent selected from the group consisting of 4 to 19% of ZrO₂ and 1to 10% of TiO₂ ; (c) from 5 to 48% of an inert liquid vehicle; and (d)from 0 to 20% of an inert filler or pigment;and firing said layer on thesubstrate to form an adherent film of the electrically conductive metalthereon.
 5. The method of claim 4 wherein the substrate is a ceramicsubstrate and the electrically conductive metal is silver.
 6. The methodof claim 5 wherein said portion of the surface of the substrate to whichthe composition is applied is pretreated by applying a silver stainthereto and firing said stain on the substrate.
 7. The method of claim 6wherein the ceramic substrate is glass.